[Sqlalchemy-tickets] Issue #4156: Error when selectin eager loading on polymorphic subtypes (zzzeek/sqlalchemy)

[Theron L. <iss...@bi...>]

New issue 4156: Error when selectin eager loading on polymorphic subtypes
https://bitbucket.org/zzzeek/sqlalchemy/issues/4156/error-when-selectin-eager-loading-on

Theron Luhn:

I'm attempting to use selectin eager loading on polymorphic subtypes using the strategy laid out here:  http://docs.sqlalchemy.org/en/latest/orm/inheritance_loading.html#eager-loading-of-specific-or-polymorphic-subtypes

Here's some example code:


```
#!python

from sqlalchemy import String, Integer, Column, create_engine, ForeignKey, inspect
from sqlalchemy.orm import relationship, Session, subqueryload, selectinload
from sqlalchemy.ext.declarative import declarative_base


Base = declarative_base()


class Company(Base):
    __tablename__ = 'company'
    id = Column(Integer, primary_key=True)
    name = Column(String, nullable=False)
    employees = relationship('Employee')


class Employee(Base):
    __tablename__ = 'employee'
    id = Column(Integer, primary_key=True)
    type = Column(String, nullable=False)
    name = Column(String, nullable=False)
    company_id = Column(Integer, ForeignKey('company.id'), nullable=False)

    __mapper_args__ = {
        'polymorphic_on': 'type',
        'with_polymorphic': '*',
    }


class Programmer(Employee):
    __tablename__ = 'programmer'
    id = Column(Integer, ForeignKey('employee.id'), primary_key=True)
    languages = relationship('Language')

    __mapper_args__ = {
        'polymorphic_identity': 'programmer',
    }


class Manager(Employee):
    __tablename__ = 'manager'
    id = Column(Integer, ForeignKey('employee.id'), primary_key=True)
    something = Column(String)

    __mapper_args__ = {
        'polymorphic_identity': 'manager',
    }


class Language(Base):
    __tablename__ = 'language'
    id = Column(Integer, primary_key=True)
    programmer_id = Column(
        Integer,
        ForeignKey('programmer.id'),
        nullable=False,
    )
    name = Column(String, nullable=False)


engine = create_engine('postgresql://localhost:5432/sa', )
Base.metadata.drop_all(engine)
Base.metadata.create_all(engine)
db = Session(engine, enable_baked_queries=False)


company = Company(
    id=1,
    name='Foobar Corp',
    employees=[Programmer(
        id=1,
        name='John Smith',
        languages=[Language(id=1, name='Python')],
    ), Manager(
        id=2,
        name='Foo',
        something='foo',
    )],
)
db.add(company)
db.flush()
db.expunge_all()

company = db.query(Company).filter(
    Company.id == 1,
).options(
    selectinload(Company.employees.of_type(Programmer)).selectinload(Programmer.languages),
).one()
print(company.employees)
```

This results in:

```
Traceback (most recent call last):
  File "scratchpad/sqlalchemy_polymorphic_loading_bug.py", line 86, in <module>
    selectinload(Company.employees.of_type(Programmer)).selectinload(Programmer.languages),
  File "/Users/luhn/Code/revenue/.env/lib/python3.6/site-packages/sqlalchemy/orm/query.py", line 2837, in one
    ret = self.one_or_none()
  File "/Users/luhn/Code/revenue/.env/lib/python3.6/site-packages/sqlalchemy/orm/query.py", line 2807, in one_or_none
    ret = list(self)
  File "/Users/luhn/Code/revenue/.env/lib/python3.6/site-packages/sqlalchemy/orm/loading.py", line 97, in instances
    util.raise_from_cause(err)
  File "/Users/luhn/Code/revenue/.env/lib/python3.6/site-packages/sqlalchemy/util/compat.py", line 203, in raise_from_cause
    reraise(type(exception), exception, tb=exc_tb, cause=cause)
  File "/Users/luhn/Code/revenue/.env/lib/python3.6/site-packages/sqlalchemy/util/compat.py", line 187, in reraise
    raise value
  File "/Users/luhn/Code/revenue/.env/lib/python3.6/site-packages/sqlalchemy/orm/loading.py", line 85, in instances
    post_load.invoke(context, path)
  File "/Users/luhn/Code/revenue/.env/lib/python3.6/site-packages/sqlalchemy/orm/loading.py", line 750, in invoke
    self.load_keys, *arg, **kw)
  File "<string>", line 1, in <lambda>
  File "/Users/luhn/Code/revenue/.env/lib/python3.6/site-packages/sqlalchemy/orm/strategies.py", line 1966, in _load_for_path
    lambda x: x[0]
  File "/Users/luhn/Code/revenue/.env/lib/python3.6/site-packages/sqlalchemy/orm/strategies.py", line 1958, in <dictcomp>
    data = {
  File "/Users/luhn/Code/revenue/.env/lib/python3.6/site-packages/sqlalchemy/orm/loading.py", line 97, in instances
    util.raise_from_cause(err)
  File "/Users/luhn/Code/revenue/.env/lib/python3.6/site-packages/sqlalchemy/util/compat.py", line 203, in raise_from_cause
    reraise(type(exception), exception, tb=exc_tb, cause=cause)
  File "/Users/luhn/Code/revenue/.env/lib/python3.6/site-packages/sqlalchemy/util/compat.py", line 187, in reraise
    raise value
  File "/Users/luhn/Code/revenue/.env/lib/python3.6/site-packages/sqlalchemy/orm/loading.py", line 85, in instances
    post_load.invoke(context, path)
  File "/Users/luhn/Code/revenue/.env/lib/python3.6/site-packages/sqlalchemy/orm/loading.py", line 750, in invoke
    self.load_keys, *arg, **kw)
  File "<string>", line 1, in <lambda>
  File "/Users/luhn/Code/revenue/.env/lib/python3.6/site-packages/sqlalchemy/orm/strategies.py", line 1987, in _load_for_path
    state.get_impl(self.key).set_committed_value(
  File "/Users/luhn/Code/revenue/.env/lib/python3.6/site-packages/sqlalchemy/orm/state.py", line 423, in get_impl
    return self.manager[key].impl
KeyError: 'languages'
```

This only happens when a Manager object is in the employees relationship.  If employees only contains programmers, it works fine.

This only happens when using selectinload, using subqueryload works fine.

I'm running the current master branch (1.2.0 doesn't work due to #4153)